It has been known since 1968-70 that contrast echocardiography can be used to delineate intracardiac structures, assess valvular competence, demonstrate intracardiac shunts, and identify pericardial effusion. (Gramiak and Shah, 1968; and Feigenbaum, et al., 1970.) Ultrasonic imaging of the heart potentially has important advantages of convenience, safety, and reduced cost over present diagnostic procedures, such as angiography, which requires the use of radio-opaque dyes for X-ray imaging, or the use of radio-nuclide imaging agents for radio-imaging. However, progress in practical applications of ultrasonic imaging has been delayed by the lack of effective clinically-usable imaging agents.
Ultrasonic imaging utilizes an ultrasonic scanner to generate and receive sound waves. The scanner is placed on a body surface overlying the area to be imaged, and sound waves are directed toward that area. The scanner detects reflected sound waves and translates that data into images. When ultrasonic energy is transmitted through a substance, the acoustic properties of the substance depend upon the velocity of the transmission and the density of the substance. Changes in the substance's acoustic properties (e.g., variations in acoustic impedance) are most prominent at the interfaces of different substances, such as a liquid-solid or liquid-gas interface. Consequently, when ultrasonic energy is directed through media, changes in acoustic properties will result in more intense sound reflection signals for detection by the ultrasonic scanner.
Ultrasonic imaging agents can consist of small solid or gaseous particles which, when injected in the circulatory system, provide improved sound reflection and image clarity. Microbubble-type imaging agents consist of minute bubbles of a gas (usually air) which are dispersed in a carrier liquid for parenteral injection. The "microbubbles" are carried by the circulatory system to the organ being imaged.
It has been proposed to form a dispersion of air microbubbles in a warm aqueous gelatin solution, and cooling the solution to a solidification temperature to trap the microbubbles. For administration, the gelled dispersion is to be warmed until it liquifies, and parenterally administered with the microbubbles dispersed in the liquified gelatin. (Tickner, et al. U.S. Pat. No. 4,276,885;- and Tickner, et al., National Technical Information Service Report HR62917-1A, April, 1977).
Gelatin-trapped microbubbles on introduction into the bloodstream have a short life-time. They rapidly dissipate. Another disadvantage is that the microbubbles are too large to pass through capillary beds, and are therefore not suitable for heart imaging by peripheral intravenous administration.
The discovery by Dr. Steven B. Feinstein of sonication-produced microbubble imaging agents represented an important advance in this art. Using viscous aqueous solutions, such as 70% sorbitol or dextrose, Dr. Feinstein produced a dispersion of microbubbles by high energy sonication of the solutions. The resulting microbubbles had sizes less than 10 microns, and were capable of passing through capillary beds. The persistence of the microbubbles, although of the order of a few minutes, permitted the imaging agent to be pre pared and administered intravenously for heart imaging. (Feinstein, et al., 1984; and Feinstein U.S. Pat. No. 4,572,203.)
Subsequently, Dr. Feinstein sought to improve the persistence of the microbubbles. He found that by sonication of a heat-sensitive protein, such as albumin, microbubbles of improved stability were obtained. (See Feinstein, PCT Application WO No. 84/02838, corresponding to allowed U.S. application Ser. No. 805,975, filed Dec. 5, 1985, now U.S. Pat. No. 4,718,433). Concentrations of microbubbles of 10 to 14.times.106 microbubbles per milliliter were obtained with bubble sizes from 2 to 9 microns (Keller, Feinstein, and Watson, 1987). The microbubbles persisted for 24 to 48 hours.
However, the sonication-produced albumin microbubble imaging agent of Feinstein was not sufficiently stable for commercial manufacture. Stabilities of the order of weeks or months (rather than hours or days) are required to permit an imaging agent to be manufactured at a central location and distributed to hospitals in the United States and other countries. For commercially feasible manufacture, shipment and hospital storage prior to use, a stability time of at least four weeks is needed and preferably at least eight weeks or longer.
Further, for the most effective imaging, it is desirable to have the highest obtainable concentration of microbubbles in the imaging agent. But the population of microbubbles of the desired small sizes tends to decrease with holding of the sonicated albumin solutions. The small bubble size attrition can occur either by collapse of the microbubbles, or by coalesence to oversize microbubbles. Consequently a further important objective has been to find means for increasing concentrations of microbubbles in the imaging agent. An imaging agent of very high microbubble concentration is inherently better, and a safety factor is provided. With a concentration of microbubbles higher than the minimum required for effective imaging, some loss of the microbubbles of the desired size can be accepted.